Method and apparatus to provide a low voltage reference generation

ABSTRACT

A method and apparatus to provide a low voltage reference generation. The apparatus includes a reference voltage generator to receive a first input voltage signal and output a reference voltage signal. A voltage level detector electrically coupled to the reference voltage generator to receive the reference voltage signal and also receive a second input voltage signal. The voltage level detector compares the second input voltage signal to the reference voltage signal for generating an output based on the compared signals.

FIELD OF INVENTION

[0001] The present invention relates generally to voltage generators.More particularly, the present invention relates to a method andapparatus to provide a low voltage reference generation.

BACKGROUND OF THE INVENTION

[0002] A reference voltage generator is a device that is powered by aninput voltage and outputs a reference voltage to compare with anothervoltage. Prior art reference voltage generators use constant voltageinput for supplying power to the voltage reference generation circuit.The same constant voltage input is also used for generating the voltagereference.

[0003] A problem with this approach is that current technology devicessupply different voltage values that vary with time. The current voltagereference generators are limited in their operation since they can onlyoperate under one constant voltage value and need to be redesigned for adifferent voltage value. Another problem with using same constantvoltage input for both supplying current to the circuit and generating avoltage reference is that it loads the voltage potential of the voltageinput by having to perform two functions. This results in producing avoltage reference that varies highly with distribution of input voltagethrough the circuit.

[0004] Prior art reference voltage generators also provide a constantoutput reference voltage irrespective of changes to the input voltage. Aproblem with this approach is that it is only applicable to a verylimited number of electronic devices that support the constant outputreference voltage. Thus this approach may not be modified to apply toparts that do not accept the produced constant reference voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

[0005] The features and advantages of the present invention areillustrated by way of example and not intended to be limited by thefigures of the accompanying drawings in which like references indicatesimilar elements and in which:

[0006]FIG. 1 shows an exemplary digital processing system in which thepresent invention can be implemented;

[0007]FIG. 2 illustrates a diagram to show a minimum voltage operatingrange for an electronic device according to one embodiment;

[0008]FIG. 3 shows a circuit for generating a low voltage referenceaccording to one embodiment;

[0009]FIGS. 4a and 4 b illustrate diagrams to show a minimum voltageoperating range and corresponding circuit for a 2V electronic partaccording to one embodiment; and

[0010]FIG. 5 shows a circuit for producing a voltage reference signalfor a 3V electronic part according to one embodiment.

DETAILED DESCRIPTION

[0011] A method and apparatus to provide a low voltage referencegeneration is described. Apparatus includes a reference voltagegenerator to receive a first input voltage signal and output a referencevoltage signal. A voltage level detector electrically coupled to thereference voltage generator to receive the reference voltage signal andalso receive a second input voltage signal. The voltage level detectorcompares the second input voltage signal to the reference voltage signalfor generating an output based on the compared signals.

[0012] The reference voltage generator includes an internal circuitcomprising a plurality of transistors and a voltage supply. The voltagesupply, separate from the first input voltage signal, is used to powerthe plurality of transistors. An advantage of having a separate voltagesupply is that the internal circuit draws current only from the voltagesupply and not from the first input voltage signal. This allowsconstruction of a low power consuming first input voltage signal andreduces the overall circuit power consumption.

[0013] Another advantage of having a separate voltage supply is that thefirst input voltage signal is left untouched. This allows the firstinput signal to retain most of its power and produce the referencesignal with very low variation across process and temperature skews. Yetanother advantage of having a separate voltage supply is that thereference voltage produced corresponds to changes in various first inputsignals and is highly independent of the power supply that powers thecircuit.

[0014] One of the uses for generating a reference voltage signal havinga low voltage potential is to compare it with the second input voltagesignal known as a program voltage signal or “Vpp”, of an electronicpart. Electronic parts, such as flash devices have a specified voltagerange for safe operation. The specified voltage range includes anoptimal operating range and a minimum operating range. The electronicpart will be damaged if operating below the minimum voltage operatingrange.

[0015] The function of a voltage level detector is to use the referencevoltage signal and compare it to an actual Vpp voltage level. Thevoltage level detector is to apply only Vpp values that are above theminimum operating range of the electronic part for its safe operation.Vpp values in the minimum operating range are low values, a low voltagereference is used to compare these values.

[0016]FIG. 1 shows an exemplary digital processing system 100 in whichthe present invention can be implemented. Referring to FIG. 1, digitalprocessing system 100 includes a bus or other communication means 101for communicating information, and a central processing unit (CPU) 102coupled with bus 101 for processing information.

[0017] CPU 102 includes a control unit 131, an arithmetic logic unit(ALU) 132, and several registers 133, which are used to processinformation and signals. The circuit of the present invention forgenerating a very low voltage reference may be implemented within ALU132. Furthermore, another processor 103 such as, for example, acoprocessor, can be coupled to bus 101 for additional processing powerand speed.

[0018] Digital processing system 100 also includes a main memory 104,which may be a random access memory (RAM) or some other dynamic storagedevice that is coupled to bus 101 for storing information orinstructions (program code), which are used by CPU 102 or processor 803.Main memory 104 may also be to store temporary variables or otherintermediate information during execution of instructions by CPU 102 orprocessor 103. Digital processing system 100 also includes static memory106, read only memory (ROM), and/or other static storage devices thatare coupled to bus 101, for storing static information or instructionsfor CPU 102 or processor 103. A mass storage device 107 which may be ahard, floppy, or optical disk drive can be coupled to bus 101 forstoring information and instructions for digital processing system 100.

[0019] A display 121 such as a cathode ray tube (CRT) or liquid crystaldisplay (LCD) can be coupled to bus 101. Display device 121 displaysinformation or graphics to a user. Digital processing system 100 caninterface with display 121 via display circuit 105. A keyboard input122, or other alphanumeric input device may also be coupled to bus 101for communicating information and command selections to CPU 102. Acursor control 123 such as a mouse, a trackball, or cursor directionkeys maybe coupled to bus 101 for controlling movement of an object ondisplay 121. A hard copy device 124 such as a laser printer may becoupled to bus 101 for printing information on paper, film, or someother like medium. A number of input/output devices such as a soundrecording and playback device 125 may also be coupled bus 101.

[0020]FIG. 2 illustrates a diagram 200 to show a minimum voltageoperating range for an electronic part according to one embodiment. Asdiscussed previously, an electronic part includes an optimal voltageoperating range and a minimum voltage operating range. A program voltagevalue, “Vpp”, within the electronic part's optimal voltage operatingrange may be applied to the electronic part for its optimal performance.A Vpp value lower than the optimal voltage operating range and withinthe minimum voltage operating range may also be applied to theelectronic part for its safe operation. Applying a Vpp value lower thanthe minimum voltage operating range damages the electronic part. Avoltage reference is selected within the minimum voltage operating rangefor comparing actual Vpp voltage level values and ensuring that onlyactual Vpp voltage level values that are above the minimum voltageoperating range are applied to the electronic part.

[0021] Referring to FIG. 2, Vpp optimal and minimum voltage operatingrange values are obtained from the specifications for the electronicpart. The optimal and minimum voltage operating vary for everyelectronic part.

[0022] The Vpp minimum voltage operating range includes an upper limitvoltage 20 and a lower limit voltage 22. The electronic part may beoperated safely by applying Vpp values above the upper limit voltage 20to the electronic part. Applying Vpp values below the lower limitvoltage 22 damages the electronic part.

[0023] Vpp values that are between the upper limit voltage 20 and lowerlimit voltage 22 may also be applied to the electronic part. However itis recommended to include a safety margin by applying Vpp values thatare above the lower limit voltage 22 for safe operation of theelectronic part.

[0024] If Vpp is at or above the upper limit voltage 20 then theelectronic part to which Vpp is being applied is unlocked and Vpp isapplied to the electronic part. If Vpp is at or below the lower limitvoltage 22 then the part is locked and Vpp is not applied to theelectronic part. Alternatively, if Vpp value is below the safety margin,then the part is locked and Vpp is not applied to the electronic part.

[0025] “V_(ref)” is a low voltage reference signal that is selectedwithin the minimum voltage operating range for comparing actual Vppvoltage level values. A V_(ref) value may be selected at any pointbetween the upper limit voltage 20 and the lower limit voltage 22.Typically, a V_(ref) value is selected at a midpoint 26 between theupper limit voltage 20 and the lower limit voltage 22. Midpoint 26 actsas the safety margin and is used for making a decision whether to lockor unlock the part in advance of reaching the lower limit voltage 22.Thus, a Vpp value above the selected V_(ref) value is applied to theelectronic part and a Vpp value below the selected V_(ref) value is notapplied to the electronic part.

[0026]FIG. 3 shows a circuit 300 for generating the low voltagereference signal according to one embodiment. Referring to FIG. 3,circuit 300 includes a plurality of transistors M1-Mn that are part of areference voltage generator, a first input signal “V1”, a voltagereference signal “V_(ref)”, a voltage level detector, a second voltageinput signal “Vpp”, and a final output. For purposes of explanation,transistors M1-Mn are depleted mode negative-channel metal Oxidesemiconductors (NMOS) transistors. A depleted mode NMOS transistor has athreshold voltage between −150 millivolts and +150 millivolts.Alternatively, transistors M1-Mn may be implemented with other types oftransistors. Combined circuitry 300 may be a part of ALU 132 used by CPU102.

[0027] The inputs, outputs, and associated circuitry for the circuit 300will now be described. Voltage potential V1 is generated through use ofcircuitry or a voltage generating source. The voltage potential at V1 istypically 1.33V for standard electronic part such as a flash device.Alternatively, the voltage potential at V1 varies with the type ofelectronic part to which it is being applied.

[0028] A voltage reference generator 30 is coupled to the voltagepotential V1 to receive the first input voltage signal. The voltagereference generator 30 includes an internal circuit comprising aplurality of transistors M1-Mn. The transistors are electrically coupledin series, have the same size within a tolerance, contain the sameelectrical criteria, and are in a saturation mode. Alternatively, thetransistors may be coupled in any combination to produce the selectedVref that is within the electronic part's minimum voltage operatingrange.

[0029] The drain of transistor M1 is coupled to a voltage potential“Vcc”. Vcc is a voltage supply for the circuit 300, which provides asaturation current for transistors M1-Mn. The gate of transistor M1 iscoupled to V1 to receive the first input voltage signal, and the sourceof transistor M1 is coupled to drain of transistor M2.

[0030] The gate and drain of transistor M2 is coupled to a voltage leveldetector 32 for outputting the selected Vref. Alternatively, voltagelevel detector 32 is coupled to gate and drain of any transistor fromthe plurality of transistors to output the selected V_(ref). Forexample, if the voltage potential between the drain of transistor M2 andground is similar to the selected V_(ref), then voltage level detector32 is coupled to M2. However if selected V_(ref) value is lower thanvoltage potential between the drain of transistor M2 and ground, thenthe voltage level detector is coupled to a transistor Mn that has avoltage potential lower than M2 and similar to the selected V_(ref).

[0031] The source of transistor M2 is coupled to either drain oftransistor Mn or ground depending upon number of transistors required.The number of transistors required depends upon voltage potential V1 andselected V_(ref). For example for a given V1 if two transistors M1, andM2 are sufficient to produce a voltage potential across at least onetransistor that is similar to the selected V_(ref), then source oftransistor M2 is coupled to ground. However if the selected V_(ref) islower than voltage potential across M1 and M2, then N number oftransistors are added to produce the selected V_(ref). In such case,source of transistor M2 is coupled to drain of transistor Mn, source oftransistor Mn is coupled to ground, and gate of Mn is coupled to sourceof M2. FIG. 4b shows a circuit including two transistors, and FIG. 5shows a circuit including more than two transistors.

[0032] The voltage level detector 32 coupled to transistor M2 is adifferential amplifier. Alternatively, the voltage level detector 32 canbe other type of circuitry that performs the same function as that of adifferential amplifier. The negative terminal of the differentialamplifier 32 receives V_(ref). The positive terminal of the differentialamplifier 32 is coupled to Vpp for receiving the second input voltagesignal, also known as actual Vpp voltage signal. The differentialamplifier has an internal circuit, which compares the received Vpp issignal with the received V_(ref).

[0033] An output 34 is coupled to the differential amplifier 32. Theoutput 34 is produced by the differential amplifier as a result of theVpp comparison made using V_(ref). The output is a “1” if the receivedVpp signal is above the received V_(ref) value, and a “0” if thereceived Vpp signal is below the received V_(ref) value. An output of“1” results in the differential amplifier 32 unlocking the electronicpart and applying the received Vpp to the electronic part. A value of“0” results in the differential amplifier 32 locking the electronic partand preventing Vpp from being applied to the electronic part, therebysecuring the electronic part from being damaged.

[0034] In one embodiment and one cycle of operation, V_(ref) is selectedfor an electronic part that is within the electronic part's Vpp minimumvoltage operating range. A circuit 300 is designed to receive a firstinput signal and produce the determined V_(ref) signal. The circuit 300includes a number of transistors, wherein the number is selected suchthat voltage potential across at least one transistor is similar to theselected V_(ref). The circuit 300 also includes a voltage supply Vccthat has a higher voltage potential than the first input signal. Voltagesupply Vcc provides a saturation current to the transistors in thereference voltage generator 30 and places the transistors in asaturation mode.

[0035] A transistor is in a saturation mode when the gate and drain ofthe transistor is coupled together. When a transistor is in a saturationmode, the voltage between the drain and the source of the transistor isgreater than or equal to the difference between the voltage potentialbetween the gate and source of the transistor and its threshold voltageas indicated in equation (1) below:

Vds≧Vgs−Vt  (1)

[0036] Where Vds=Voltage between drain and source of a transistor,Vgs=Voltage between gate and source of the transistor, and Vt=Transistorthreshold voltage.

[0037] Alternatively, a transistor is in saturation mode when its gatereceives an input voltage, and drain receive a Vcc voltage, where theVcc voltage potential is higher than the difference between the inputvoltage and the threshold voltage.

[0038] The transistors in saturation receive saturation current fromVcc. The relationship between the saturation current and voltagepotential across a transistor in saturation mode is explained inequation (2) shown below:

I _(ds)=(β/2)*(Vgs−Vt)²  (2)

[0039] Where β is a constant determined by equation (3) shown below:

β=μ*C _(ox) *W/L  (3)

[0040] Where μ=Effective surface mobility of carrier inside a channel ofa transistor, W=Width of the transistor, and L=Length of the transistor.

[0041] Equation (2) may be solved to getting the voltage drop acrosseach transistor as shown in equation (4) shown below:

Vgs=[2/β*Sqrt(Ids)]+Vt  (4)

[0042] Where Vt is a constant threshold voltage.

[0043] Since the width, length, effective surface mobility, allelectrical criteria, and saturation current Ids for all the transistorsis the same, the voltage drop Vgs across each transistor is also thesame. Thus the input voltage is equally divided over the number oftransistors in the reference voltage generator 30.

[0044] The transistor having a voltage potential similar to V_(ref) iscoupled to the voltage level detector 32. The voltage level detector 32compares Vpp to V_(ref) and applies Vpp values that are above V_(ref) tothe electronic part.

[0045] In another embodiment and another cycle of operation, a circuit300 is designed to produce any desired V_(ref) voltage signal. Thecircuit includes the plurality of transistors M1-Mn that are insaturation mode and coupled to a voltage supply for receiving asaturation current. A transistors having the desired V_(ref) voltagebetween its source and ground is coupled to a differential amplifier.V_(ref) signal is used to compare any second input signal received bythe differential amplifier. The comparison results in allowing thereceived second input signal to pass as the output signal if thereceived second input signal has a voltage potential that is higher thanthe V_(ref) signal.

[0046]FIGS. 4a and 4 b illustrate diagrams to show a minimum voltageoperating range for a 2V electronic part and a corresponding circuitaccording to one embodiment.

[0047] Referring to FIG. 4a, the optimal voltage operating range for a2V electronic part is from 1.65V to 1.95V. The minimum voltage operatingrange for a 2V electronic part is from 0.4V-0.9V. The minimum voltageoperating range for a 2V electronic part includes an upper limit 40having a voltage potential of 0.9V and lower limit 42 having a voltagepotential of 0.4V.

[0048] Applying a Vpp above the upper limit 40 of 0.9V operates the 2Velectronic part safely. Applying a Vpp below the lower limit 40 of 0.4Vdamages the 2V electronic part. As shown in FIG. 4a, a V_(ref) range 44exists between 0.4-0.9V. A V_(ref) may be selected at any point withinthe V_(ref) range 44. The V_(ref) range 44 includes the lowest valuesallowed by the electronic part specifications for safe part operation. AV_(ref) may be selected at a midpoint 46 for the 2V electronic part.Selecting a midpoint 46 having a value of 0.65V as the V_(ref) providesa safety margin by locking the part before Vpp reaches 0.4V lower limitvoltage. Alternatively, a V_(ref) may be selected at a point higher thanmidpoint and closer to the upper limit 40 thereby achieving a highersafety margin.

[0049]FIG. 4b shows a circuit designed for the standard electronic partsuch as a 2V flash device that achieves a V_(ref) of approximately0.65V. The circuit includes a voltage reference generator 440 comprisingtwo transistors M1 and M2 that are connected in series, of same size andelectrical criteria, and in saturation.

[0050] The input voltage potential V1 of 1.33V is distributed over thetwo transistors to achieve a voltage potential of 0.66V across eachtransistor. Transistor M2 is coupled to differential amplifier 442 tooutput a V_(ref) value of 0.66V. The differential amplifier 442 uses theV_(ref) of 0.66V to compare the current Vpp. If Vpp is above 0.66V thenVpp is applied to the 2V device. However if Vpp is less than 0.66V thenthe part is locked out and Vpp is not applied.

[0051]FIG. 5 shows a circuit for producing a voltage reference signalfor a 3V electronic part according to one embodiment. As previouslydiscussed, the midpoint for a 3V electronic part can be determinedsimilar to the 2V electronic part from specifications for the 3Velectronic part. The midpoint for a 3V electronic part fromspecification can be selected at 1.8V. Which means that if the Vpp inthe current circuit is above 1.8V then the part stays unlocked, and ifits below 1.8V then part is locked.

[0052] One embodiment of designing a circuit to produce a referencesignal V_(ref) for comparing current Vpp is further described. A voltagereference generator 50 with three transistors M1-M3 is designed. Thegate of the first transistor M1 is coupled to voltage potential V1 toreceive the first input voltage signal of 1.33V. The received firstinput signal of 1.33V is distributed equally across transistors M1, M2,and M3 at 0.443V per transistor.

[0053] The drain of M1 is coupled to voltage supply Vcc and the sourceof M1 is is coupled to drain of M2. The gate and drain of M2 is coupledto differential amplifier 52 to output a V_(ref). The source of M2 iscoupled to drain of M3. The source of M3 is coupled to ground, and gateof M3 is coupled to source of M2.

[0054] Since voltage potential V1 is equally distributed at {fraction(1/3)} per transistor, the transistor M2 is coupled to the differentialamplifier 52 at point “A” which is a V_(ref) output, to produce aV_(ref) output of 0.886V.

[0055] The negative terminal of a differential amplifier 52 is coupledto transistor M2 to receive the produced V_(ref). The positive terminalof the differential amplifier 52 is coupled to resistor R2. Resister R2is coupled to resistor R1 in series which is coupled to voltagepotential Vpp to receive the second input voltage signal. Since Vppvalue of approximately 1.8 is estimated from specification for a 3Vpart, the two resistors in series drop Vpp voltage by half producing anestimated 0.9V. Using of resistors and dropping estimated Vpp voltage to0.9V allows a close comparison with the V_(ref) 0.886.

[0056] Differential amplifier 52 is coupled at point “A” to achieve avoltage potential V_(ref) similar to the estimated Vpp of 0.9.Alternatively, if a lower value of Vpp is estimated then differentialamplifier would be connected at point “B” to produce a lower V_(ref),i.e. to the gate and drain of M3 thereby achieving a V_(ref) of 0.443instead of 0.886.

[0057] The differential amplifier 52 compares V_(ref) of 0.886 with theactual Vpp voltage level. A compared Vpp value above 0.886V results inan output 54 of “1” unlocking and applying the Vpp to the electronicpart. A compared Vpp value below 0.886V results in an output 54 of “0”locking and not applying the Vpp to the electronic part.

[0058] These and other embodiments of the present invention may berealized in accordance with these teachings and it should be evidentthat various modifications and changes may be made in these teachingswithout departing from the broader spirit and scope of the invention.The specification and drawings are, accordingly, to be regarded in anillustrative rather than restrictive sense and the invention measuredonly in terms of the claims.

What is claimed is:
 1. An apparatus comprising: a reference voltagegenerator to receive a first input voltage signal and output a referencevoltage signal; and a voltage level detector electrically coupled to thereference voltage generator to receive the reference voltage signal andto receive a second input voltage signal for comparing the second inputvoltage signal to the reference voltage signal and generating an outputbased on the compared signals.
 2. The apparatus of claim 1, wherein thevoltage level detector further comprises: a first terminal to receivethe second input voltage signal; a second terminal to receive thereference voltage signal; and circuitry electrically coupling the firstterminal and the second terminal to an output.
 3. The apparatus of claim1, wherein the reference voltage signal has a voltage potential that islower than the voltage potential of the first input voltage signal. 4.The apparatus of claim 1, wherein the voltage level detector is adifferential amplifier.
 5. The apparatus of claim 1, wherein thereference voltage generator further comprises: a plurality oftransistors, wherein all the transistors are electrically coupled inseries, the gate and drain of at least one transistor electricallycoupled to a differential amplifier, the source of at least onetransistor electrically coupled to a ground, and at least one transistorwith its drain electrically coupled to a voltage supply and its gatecoupled to an input to receive the first input voltage signal.
 6. Theapparatus of claim 1, wherein the reference generator and the voltagelevel detector are part of a flash memory device.
 7. The apparatus ofclaim 1, wherein the reference voltage generator further includes aplurality of transistors.
 8. The apparatus of claim 7, wherein thereference voltage signal has a voltage potential that is the same as avoltage potential across each of the plurality of transistors.
 9. Theapparatus of claim 7, wherein the plurality of transistors are in asaturation mode.
 10. The apparatus of claim 7, further including avoltage supply for providing a saturation current to the plurality oftransistors.
 11. The apparatus of claim 7, wherein current for theplurality of transistors is supplied by a voltage supply.
 12. Theapparatus of claim 7, wherein the reference voltage signal may becoupled to the gate and drain of any one of the transistors from theplurality of transistors.
 13. The apparatus of claim 7, wherein each ofthe transistors in the plurality of transistors includes similarelectrical criteria.
 14. A method for generating a low reference voltagecomprising: receiving a first input voltage signal; processing thereceived first input voltage signal to generate a reference voltagesignal, wherein the reference voltage signal has a voltage potentialthat is lower than the voltage potential at the first input voltagesignal; and comparing the reference voltage signal to a second inputvoltage signal for generating an output.
 15. The method of claim 14,further comprising using a voltage supply to supply current to a circuitthat produces the reference voltage signal.
 16. The method of claim 14,wherein processing further comprising: using a plurality of transistorscoupled in series; applying the first input voltage signal to at leastone transistor; and distributing the voltage potential from the firstinput voltage signal equally over the plurality of transistors to outputthe reference voltage signal such that the reference voltage signal hasthe same voltage potential as the voltage potential across at least oneof the transistors.
 17. The method of claim 14, further comprisingproviding a saturation current to a plurality of transistors.
 18. Themethod of claim 14, wherein comparing further comprising: receiving thereference voltage signal; receiving the second input voltage signal; anddetermining whether the second input voltage signal is higher or lowerthan the reference voltage signal.
 19. The method of claim 18, furthercomprising applying the second input voltage signal to an electronicpart if the voltage potential of the second input voltage signal ishigher than the voltage potential of the reference voltage signal.
 20. Adigital processing system comprising: a memory; and a processor toreceive a first input voltage signal, to process the received firstinput voltage signal to generate a reference voltage signal, wherein thereference voltage signal has a voltage potential that is lower than thevoltage potential at the first input voltage signal, and to compare thereference voltage signal to a second input voltage signal for generatingan output.
 21. The digital processing system of claim 20, wherein theprocessor is to use a voltage supply to supply current to a circuit thatproduces the reference voltage signal.
 22. The method of claim 20,wherein the processor is to use a plurality of transistors coupled inseries, is to apply the first input voltage signal to at least onetransistor, and is to distribute the voltage potential from the firstinput voltage signal equally over the plurality of transistors to outputthe reference voltage signal such that the reference voltage signal hasthe same voltage potential as the voltage potential across at least oneof the transistors.
 23. The method of claim 20, wherein the processor isto receive the reference voltage signal, is to receive the second inputvoltage signal, and is to determine whether the second input voltagesignal is higher or lower than the reference voltage signal.
 24. Themethod of claim 23, wherein the processor is to apply the second inputvoltage signal to an electronic part if the voltage potential of thesecond input voltage signal is higher than the voltage potential of thereference voltage signal.